Such a method is known from European Patent Application published on Mar. 16, 1983 under No. 74215. In this case, the masking is constituted by a thin first layer of silicon oxide, a thicker second layer of polycrystalline silicon and a third layer of photosensitive material which has substantially the same thickness as the second layer. After the implantation treatment, a metal layer is provided, of which the part located on the masking is removed by dissolving the photosensitive layer. Subsequently, the polycrystalline silicon layer of the masking is removed. The metal pattern thus obtained forms a complementary mask, which accurately masks those parts of the semiconductor body which were subjected before to the implantation treatment. By using a second implantation treatment with a doping of the opposite type, doped regions of opposite conductivity types are obtained in the semiconductor body, which accurately adjoin each other along the edge of the masking or the metal pattern. During the first implantation treatment, boron is implanted at an implantation energy of 120 keV. For the second implantation treatment, phosphor is used at an implantation energy of 200 to 300 keV.
In the method known from the publication "Scalable retrograde p-well CMOS technology", International Electron Devices Meeting 1981, Technical Digest, p. 346-349, an implantation treatment is carried out for doping the active regions for forming a so-called "retrograde p-well". Boron ions are implanted at a dose of 2.10.sup.13 per cm.sup.2. The penetration depth of these ions, i.e. the depth under the surface at which after the implantation treatment the concentration of the implanted dopant practically has a maximum, is in the relevant active regions about 1 /.mu.m. The field insulation consists of a pattern of oxide, which is obtained in a usual manner by local oxidation of the semiconductor body. The thickness of the pattern of field oxide, i.e. the first layer thickness, is about 0.8 /.mu.m. During the implantation treatment, a masking (not shown) is present on the semiconductor body, which masking covers a number of the active regions and has openings at the area of other active regions, within which openings besides the relevant active region also an adjacent part of the field oxide surrounding this region is always disposed. The implanted dopant penetrates within the openings both in the active regions and under the part of the field oxide not covered by the masking into the semiconductor material of the semiconductor body. On the assumption that the penetration depth (the so-called "range") in silicon dioxide is approximately equal to that in silicon, the maximum doping concentration, which in the active regions lies about 1 /.mu.m under the semiconductor surface, will be situated under the field oxide at a depth of about 0.2 /.mu.m. Consequently, after the treatments at high temperature required for the manufacture of the complete integrated circuit, the surface concentration of the doping of the "p-well" in the parts located under the field oxide is comparatively high, as a result of which a specific doping treatment for obtaining p-type channel stopper zones adjoining the field oxide may not be required. In this case, as a result of the comparatively high surface concentration under the field oxide, the parasitic threshold voltage for channel formation under this oxide in the p-well is already sufficiently large.
In the afore-mentioned known methods, the accelerated ions penetrate within the openings, through the parts of the field oxide exposed there, into the subjacent semiconductor material. During the implantation treatment, the field oxide therefore does not mask against the ions used in this treatment. This is due to the fact that silicon dioxide is for the accelerated ions not a really dense material especially as compared with silicon. Silicon dioxide is a semi-masking material. In the present description, this material is to be understood to mean a material having an average penetration depth or "range" of the accelerated ions to be used which is comparable with or is at least practically of the same order as the average penetration depth of these ions in the semiconductor material to be masked.
It is usual in implantation treatments to use a patterned photolacquer layer as a mask. The photolacquers (resists) usual in the semiconductor technology are semimasking materials, however. In order that it can be used as a mask, the thickness of the layer of semi-masking material, i.e. the second thickness, has to be larger than the average penetration depth of the accelerated ions to be used in this material in order that the accelerated ions can be stopped completely. This means inter alia that during implantation treatments at a high implantation energy and with a masking of semi-masking material, as in the present method, the masking must have a considerable thickness. In such comparatively thick masking photolacquer layers, the desired masking pattern can often be provided only with difficulty with the desired accuracy.